As companion animals age and pass through various life stages from in utero to the geriatric state, nutrient requirements change along with the manner in which nutrients are utilized by the various organ systems in the body. From the regulatory perspective, recognized life stages include maintenance, growth, and gestation/lactation. Other important life stages include in utero, the neonate, and the senior/geriatric state. Age affects digestive physiological properties, too, and factors such as gut microbiota, digestive hormones, gut morphology, gut immunity, and nutrient digestibility are modified as the animal becomes older. Each of the nutrients is affected in some manner by age, some more than others. Genomic biology offers promise in helping elucidate in greater detail how nutrient utilization is affected by age of the dog and cat.